Submissive signalling in mule deer

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ANIMAL

BEHAVIOUR,

recognition. Whether or not such a process occurs, it is not required by the model, which postulates only that territory choice is guided by dialect familiarity. Because of the processes of learning that establish dialect differences, this will ensure an increased probability of encountering relatives. Such an enhanced likelihood is sufficient to allow selection for an ESS ensuring a greater expected return to the individuals in conflict. This paper was written while in receipt of support from the Medical Research Council of Great Britain. MICHEL TREISMAN

Department of Experimental Psychology, University of Oxford, Oxford OXl 3UD, England. References Greenwood, P. J. & Harvey, P. H. 1976. The adaptive significance of variation in breeding area fidelity of the blackbird (Turdas merula L.). J. Anita. Ecol., 45, 887-898. Harris, M. A. & Lemon, R. E. 1974. Songs of song sparrows: reactions of males to songs of different localities. Condor, 76, 33-44. Lemon, R. E. 1967. The response of cardinals to songs of different dialects. Anim. Behav., 15, 538-545. Maynard Smith, J. 1974. The theory of games and the evolution of animal conflicts. J. theor. Biol., 47, 209221. Milligan, M. M. & Verner, J. 1971. Inter-populational song dialect discrimination in the white-crowned sparrow. Condor, 73, 208-213. Nottehobm, F. 1969. The song of the chingolo, Zonotrichia capensis, in Argentina : description and evaluation of a system of dialects. Condor, 71, 299-315. Nottebohm, F. 1970. Ontogeny of bird song. Science, N.Y., 167, 950-956. Nottebohm, F. 1972. The origins of vocal learning. Am. Nat., 106, 116--140. Trainer, J. M. 1980. Comments on a kin association model of bird song dialects. Anim. Behav., 28, 310-311. Treisman, M. 1977. The evolutionary restriction of aggression within a species: a game theory analysis. J. Math. Psychol., 16, 167-203. Treisman, M. 1978. Bird song dialects, repertoire size, and kin association. Anim. Behav., 26, 814-817. Weeden, J. S. & Falls, J. B. 1959. Differential responses of male ovenbirds to recorded songs of neighboring and more distant individuals. Auk, 76, 343-351.

(Received 10 September 1979; MS. number: AS-84) Submissive Signalling in Mule Deer Submissive displays may enable a subordinate animal to remain in a particular place and to gain the benefits of staying within a group despite the presence of dominant conspecifics. It has commonly been suggested that submissive signals act as appeasement behaviours to inhibit further aggression by the victor of an agonistic encounter (see K. Lorenz, On Aggression, p. 131, London: Methuen, 1966). These submissive actions emphasize the absence of aggressive signals and often involve patterns that are a direct antithesis of aggressive displays in the species (C. Darwin, The Expression of the Emotions in Man and the Animals, p. 50, 1872; reprinted Chicago: University of Chicago Press, 1965).

28,

1

I examined whether displays performed by the loser of an agonistic encounter actually function to reduce aggressive behaviour toward the signaller during mule deer (Odocoileus hemionus californicus) antler pushing (sparring) contests. Field observations were conducted on a population of 40 deer, 10 of which were adult males, from August 1977 through November 1977 in the Chatsworth Reservoir (449 ha), Los Angeles County, California. All males participated in contests, but smaller-antlered, lowerranking bucks were involved in a higher frequency of sparring contests than were large-antlered, high-ranking males. The loser of a contest was defined as that individual which disengaged his antlers, retreated from his opponent, and did not attempt to re-initiate the sparring encounter. The behaviour of the loser for 153 sparring contests was classified with respect to the following three dichotomies: (a) the loser retreated ~< 2 m from the sparring opponent, or he retreated > 2 m; (b) the loser assumed an orientation facing the opponent after retreat, or he assumed an orientation facing away from the sparring partner; and (c) the loser held his head in an erect posture, or the loser lowered his head into a feeding position. These signal classes represent the extremes of a graded continuum of possible responses. However, intermediate positions for (b) and (c) were infrequent. The immediate action of the winner following the sparring contest was also recorded, as either further aggressive display or no further display toward the loser. The agonistic displays performed by the winner were chases, crouch threats, or antler threats as an invitation to engage in continued sparring. The behaviour of the loser and subsequent action of the winner for 153 sparring contests are shown in Table I. The effectiveness of the three signal classes in reducing aggression was analysed separately (Part A) and as combinations of submissive displays (Part B). In every case of the single component analysis the response of the winner differed significantly depending upon the action of the loser (~z

Table I. Submissive Signals following Defeat in Sparring Contests and the Subsequent Action of the Winner

Action of loser

Frequency of aggressive display by winner (%)

A. Single component analysis: Retreat ~< 2 m (N = 76) Retreat > 2 m (N = 77) x 2 = 18.5 (dr= 1 ; P < 0.005) Orientation facing (N : 15) Orientation opposite (N = 138) Xz = 9.6 (df = 1 ; P < 0.005) Head held erect (N = 95) Head lowered in feeding (N = 58) X 2 = 22.7 (df = 1 ; P < 0.005)

80 47 100 59 78 40

B. Combination analysis: > 2 > 2 > 2 > 2 ~< 2 ~< 2 ~< 2 < 2

m, m, m, m, m, m, m, m,

Opposite, lowered (N = 42) Facing, lowered (N = 0) Opposite, erect (N = 31) Facing, erect (N = 4) Opposite, lowered (N = 16) Facing, lowered (N = 0) Opposite, erect (N = 49) Facing, erect (N = 11)

31 61 100 62 81 100

SHORT COMMUNICATIONS test; P < 0.005). The latter action in each dichotomy was considered to be the more submissive signal, and resulted in a significantly lower frequency of further aggressive displays by the winner than did the alternate response (arcsin test (R. R. Sokal & F. J. Rohlf, Biometry, San Francisco: W. H. Freeman, 1969): (a) t, = 4.67; (b) t, = 5.08; and (c) t~ = 4.8, respectively; for all, P < 0.001). Part B of Table I shows that display combinations utilizing the more submissive signals resulted in a lower frequency of agonistic behaviour from the contest winner. Thus the behaviour pattern using all three of the most submissive displays ( > 2 m, Opposite, Lowered) is clearly more effective at limiting aggression than are any other combinations. Similarly, combinations with two of the three more submissive signals elicited less aggression than those with only one or none. Thus in mule deer agonistic encounters the degree of submission indicated by the behavioural display of the contest loser mediated the subsequent aggressive response of the dominant animal. Highly submissive displays less often elicited aggressive actions from the contest winner following an initial encounter. DEBRA L. KOUTN1K

Section of Neurobiology and Behavior, Langmuir Laboratory, Cornell University, Ithaca, New York 14850. (Received 27 July 1979; revised 28 September 1979; MS. number: AS-81) Polygyny and Nest Spacing in Lapland Longspurs: A Hypothesis Recent articles have emphasized the advantages of female defence of depreciable attributes of avian territories (Altmann et al. 1977). Whenever feasible, 'resident females should try to prevent settling and polygynous matings by new arrivals' (Wittenberger 1976, page 793). When exclusion of additional females is not feasible, selection should favour those females who retain most of the depreciable benefits of territories (e.g. resources, male assistance). We hypothesize that nest positioning is an important mechanism for maintaining most or all of territorial benefits. Nest placement determines the centre of the utilized area during the nestling period (Tryon 1978); hence, a nest placed in the centre of a male display territory may maximize use of the exclusive area. Also, polygynous females tend to partition territories (McLaren 1972; Wittenberger 1976); thus, a centred nest may partition most of the exclusive area to the benefit of the first female or even discourage additional females from nesting on the territory. We previously reported on territory size, polygyny and nest locations for a population of Lapland longspurs (Calcarius lapponicus) near Barrow, Alaska (Seastedt &

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MacLean 1979). We repeated these measurements at a subarctic site (Old Chevak, Alaska: site description in Mickelson 1975). Territory size in the subarctic was significantly smaller (P < 0.01, Mann-Whitney U test) and averaged 1.30 ha versus 1.76 ha at the arctic site. We found two cases of polygyny out of 11 males (18%) at the subarctic site in 1976 versus one polygynous male out of 22 males (4.5 %) studied at Barrow in 1975. A test procedure was developed to quantify nest placement with respect to the display boundaries of males. Let r be the distance from the centre of a territory to any part of its boundary. Since the territories were relatively symmetrical, the centres could be approximated by an iterative procedure of drawing lines to represent a series of diameters, and locating their point of convergence. All points at distance r/2 from the centre of the territory were then connected to create an inner polygyon with an area 0.25 that of the territory. A nest was considered centred if it was found within the inner polygon. If nest placement within the territory is random, then the probability of finding a nest within any area is directly proportional to the ratio of that area to total territory size. Thus, the probability of a nest occurring in the inner polygon is 0.25. Given a sample of N nests, the probability of observing k or more centred nests can be determined by the binomial experiment. The results (Table I) indicate that nests at Old Chevak showed a far greater than random probability of occurring in the centre fourth of territories, while nests at Barrow did not. Mean nest distances for contiguous territories were 85 m and 106 m at the subarctic and arctic sites, respectively. The coefficient of variation (sD/mean nearest nest distance) was 0.16 at Old Chevak but 0.42 at Barrow. Thus, nests were more evenly spaced at the subarctic site, which again suggests centred nests. Why are nests not centred at Barrow? One possibility is that a higher frequency of polygyny exists at Old Chevak. If the potential for polygyny is less at Barrow, the selective pressure for nest centring is reduced. This might occur if (1) there is a shortage of females in the Barrow population but not at Old Chevak, and/or (2) resources are more abundant over a longer period of time to support polygynous nestings at the subarctic site. Alternatively, the tendency to centre nests might be suppressed at Barrow by strong selection favouring nests placed near productive feeding sites. Since the establishment of territories occurs well before the peak of snow melt (Seastedt & MacLean 1979), males may fail to centre territories on the most productive feeding habitat. In a heterogeneous environment, female fitness may be best served by placing the nest in the best feeding habitat, since this will improve her own efficiency and that of the male (should he assist) in feeding nestlings.

Table I. Nest Locations with Respect to Male Territory Boundaries Site

Number of Nests Sample size (N)

Barrow (1975) Old Chevak (1976)

18 13

Probability*

Centred (k)

Near perimeter

4 8

14 5

0.694 0.006

*Probability of observing k or more nests located in the centre 0.25 area of the territories, given N samples.